The present invention relates to a method and an apparatus for manufacturing a thin-film photovoltaic conversion device built on a flexible substrate, such as a solar cell, which can convert light energy into electric energy.
Recently, thin-film photovoltaic conversion devices which use thin-film semiconductors of silicon-based materials, such as amorphous silicon and amorphous silicon alloys, as thin-film photovoltaic conversion devices have attracted attention. Cost reduction through mass production is a significant issue in manufacturing solar cells, which are representative of thin-film photovoltaic conversion devices, and, naturally, enhancement of production quantity per unit time is always a key objective. Conventional rigid glass plates and stainless steel sheets generally used for the substrates of thin-film photovoltaic conversion devices make the process of loading and unloading the substrates to a vacuum device complex, and also complicate their mounting on, and removal from, substrate holders.
In search of ways to reduce the manufacturing time, much has been expected from devices which use resin or similar materials for substrates because of their potential for reducing the substrate cost and making it easier to use solar cells. For these reasons, a "roll-to-roll" system has been developed, in which a belt-shaped flexible substrate in the form of a roll is placed in a loading chamber, and a thin-film photovoltaic conversion device with a multi-layer construction is repeatedly formed on the belt-shaped substrate as the substrate passes through various reaction chambers. Such a system is disclosed by K. Suzuki et al. in the "Technical Digest of the International PVSEC-1" (1984), p. 191, or by S. R. Ovshinsky et al. in the "Technical Digest of the International PVSEC-1" (1984), p. 577, for example.
FIG. 2 shows a conventional film-forming apparatus that can form a photovoltaic conversion layer having a pin construction. A belt-shaped flexible substrate (1) unrolled from a loading roller (2) in a loading chamber (20) passes a transporting roller (4) and goes into a loading chamber (30) where it is continuously wound around an unloading roller (3). When the substrate is positioned within a p-layer forming chamber (31), a two-layer is formed on the surface of the flexible substrate (1) as a result of a reaction in which gas is decomposed by plasma generated between a high-voltage electrode (51) and an earth electrode (52) disposed with a substrate heater (6). Similarly, an i-layer and an n-layer are formed when the substrate passes an i-layer forming chamber (32) and an n-layer forming chamber (33), respectively. Exhaust systems (7) are connected to the loading chamber (20), each reaction chamber (31), (32) and (33), and the unloading chamber (30).
Typically, electrode layers, one of which is a transparent electrode layer, are disposed on both sides of a photovoltaic conversion layer having a pin construction. To form such electrode layers, which are conductive layers, a film-forming apparatus as shown in FIG. 3 is used. The belt-shaped flexible substrate (1) unrolled from the loading roller (2) in a film-forming chamber (39), which is connected with an exhaust tube (71) and a gas-introducing tube (70), passes the transporting roller (4) and a heating roller (60) where it is continuously wound round the unloading roller (3). Because the face of a target (53), which is made of a conductive material, is exposed to plasma generated between the target (53) and the earth electrode (52) disposed on the heating roller (60), a conductive layer is formed on the substrate surface when the substrate (1) passes the heating roller (60).
Several drawbacks exist in a film-forming device shown in FIG. 2. As a result of the flexible substrate (1) passing continuously through the reaction chambers (31), (32) and (33), air-tightness cannot be maintained sufficiently at the partition (36) separating the reaction chambers. As a result, gases from the adjacent reaction chambers intermingle. Moreover, the substrate (1) and films thereon may be damaged by friction with the partition (36) and the earth electrode (52). In addition, because the reaction chambers must be maintained at the same pressure, individual chambers cannot be controlled independently to assure the optimal pressure required for a desired film quality. Installing a preliminary chamber (35) as shown in FIG. 4, which is maintained at a low pressure via an independent exhaust system (7), between the reaction chambers (31) and (32), and also between the reaction chambers (32) and (33), may partially correct the gas-permeating problem and the pressure dependence of each reaction chamber, but the problem of substrate damage is not corrected.
FIG. 5 shows a multi-layer construction designed to improve the conversion efficiency in thin-film solar cells, which structure consists of ten layers. The bottom layer, the substrate (1), is covered with an electrode layer (29). Disposed on top of the layer (29) is a p-layer (21) having a thickness of 100 .ANG. to 200 .ANG. and consisting of amorphous silicon-carbon alloy (a-SiC) or amorphous silicon-oxygen alloy (a-SiO). Next, a buffer layer (22) having a thickness of 100 .ANG. to 200 .ANG. and consisting of amorphous silicon (a-Si), a-SiC or a-SiO is disposed on top of the layer (21). Disposed on top of the layer (22) in ascending sequential order are: an i-layer (23) having a thickness of 700 .ANG. and consisting of a-Si; an n-layer (24) having a thickness of 300 .ANG. and consisting of a-Si; a p-layer (25) having a thickness of 100 .ANG. to 200 .ANG. and consisting of a-SiC or a-SiO; a buffer layer (26) having a thickness of 100 .ANG. to 200 .ANG. and consisting of a-Si, a-SiC or a-SiO; an i-layer (27) having a thickness of 3,000 .ANG. and consisting of a-Si; and an n-layer (28) having a thickness of 300 .ANG. and consisting of a-Si.
Photovoltaic conversion devices having a multi-layer construction shown in FIG. 5 may be manufactured by using a conventional film-forming apparatus shown in FIG. 6. The apparatus has two sets of p-layer forming chambers (31), a buffer layer-forming chamber (34), an i-layer forming chamber (32), and an n-layer forming chamber (33) arranged between the loading chamber (20) and the unloading chamber (30). The apparatus shown in FIG. 6 has several major drawbacks, one of which is that, as the number of chambers traversed by the substrate increases, severity of the substrate damage increases. In addition, because the substrate travels through the film-forming chambers at a constant speed, either the lengths of the chambers must be designed to match the film-forming speeds of each film, or the film-forming speed must be adjusted in order to build a multi-layer structure having film-layers of varying film thicknesses and film-forming speeds.
If the lengths of the chambers are made dependent on the film-forming speeds, no degree of freedom can be given to the size of the apparatus, and no adjustments are possible after the apparatus is designed. If one chooses the option of adjusting the film-forming speeds, optimal film-forming speed for a given type of film may not be selected. For instance, if the n-layer (24) or (28) with a thickness of 300 .ANG. is formed in the film-forming chamber (33) in one stage while the i-layer (27) with a thickness of 3,000 .ANG. is formed in the film-forming chamber (32) in a subsequent stage, the relative lengths of the film-forming chambers must have a ratio of 1:10, or the film-forming speed must be set to 1/10, something which is extremely difficult to realize.
It is an object of the present invention to provide an improved method for manufacturing a thin-film photovoltaic conversion device.
It is another objective of the present invention to provide an improved apparatus for manufacturing a thin-film photovoltaic conversion device.
It is another object of the present invention to provide an improved method for manufacturing a thin-film photovoltaic conversion device, which method facilitates formation of films in each layer of the device under optimal pressures.
It is another objective of the present invention to provide an improved apparatus for manufacturing a thin-film photovoltaic conversion device, which apparatus forms films in each layer of the device under optimal pressures.
It is another object of the present invention to provide an improved method for manufacturing a thin-film photovoltaic conversion device, which method will not damage the flexible substrate being formed.
It is another object of the present invention to provide an improved apparatus for manufacturing a thin-film photovoltaic conversion device, which apparatus will not damage the flexible substrate being formed.
It is another object of the present invention to provide an improved method for manufacturing a thin-film photovoltaic conversion device, which method will allow a greater degree of freedom in selecting the size of the apparatus and the film-forming speed of each layer of the conversion device.
It is another object of the present invention to provide an improved apparatus for manufacturing a thin-film photovoltaic conversion device, which apparatus will allow a greater degree of freedom in selecting the size of the apparatus and the film-forming speed of each layer of the conversion device.